1. Field of the Invention
The present invention relates to an image forming apparatus which forms as a latent image a picture consisting of a plurality of pixels on the surface of a photosensitive body based on a picture signal indicating text and graphic data.
2. Description of the Related Art
Conventionally, in general, a laser printer having a semiconductor laser as a light source operates to apply a laser beam, deflectively scanned by a rotary polygonal mirror or the like, to a rotary photosensitive body for forming an electrostatic latent image corresponding to the picture on the surface of a photosensitive body. The printer then takes the steps of developing, transferring, fixing the latent image and finally recording the image on a transfer sheet of paper. The direction of scanning done by the rotary polygonal mirror is referred to as a "main scanning" direction and the direction of rotation of the photosensitive body, that is, the feeding direction of the sheet is referred to as a "sub scanning" direction. The rotary speed of the photosensitive body may be far lower than that of the rotary polygonal mirror and thus is negligible.
For forming the electrostatic latent image on the surface of the photosensitive body, the laser printer is arranged to apply light energy sufficient to form one pixel onto the photosensitive body by operating the semiconductor laser for a certain length of time; this operating time is referred to a pulse width. Such pixels forming one line are formed in the main scanning direction. Then, those lines are formed in the sub scanning direction so that the dot matrix for one sheet of paper is formed on the surface of the photosensitive body. The form of one pixel, formed on the surface of the photosensitive body by the above method, is defined depending on the recording density of a laser beam applied to the photosensitive body. As such, the form of a laser beam spot on the photosensitive body is often chosen to be elliptical by considering the far field pattern characteristics of the semiconductor laser.
In a case that, therefore, the laser spot on the photosensitive body is elliptical (where the diameter of the main scanning direction is denoted by Wx and the diameter of the sub scanning direction is denoted by Wy), assuming that an area, having a magnitude of light energy applied on the photosensitive body equal to or larger than a constant value (for example, the constant value is one [.mu.J/cm.sup.2 ], if the area having a larger magnitude of light energy being equal to or larger than a value of a [.mu.J/cm.sup.2 ] becomes an image at the final stage) is made to be a part where oblique lines are depicted as shown in FIG. 38. The light energy density of the laser beam forms a Gaussian distribution. Hence, by inputting beam-radiating conditions such as a laser output, a pulse width, a scan speed in the main scan direction, and an elliptical beam diameter as input parameters, the distribution of the light energy applied on the photosensitive body can be calculated and derived as a numerical value. The area having the larger magnitude of light energy than a [.mu.J/cm.sup.2 ] derived by the above calculation is made to be substantially elliptical.
Normally, one pixel of 300 dpi (dot per inch) is a substantially elliptic form whose diameter is about 80 to 100 .mu.m. Hence, the sloped part contained in a character or a graphic is formed to have a stepwise jaggy, which is not desirable in terms image quality. As the recording density increases, for example, it reaches about 600 dpi, the jaggy appearing on the sloped part is not so conspicuous. However, if the recording density is doubled, quadruple memory is required, resulting in greatly enhancing the cost. To overcome this shortcoming, to enhance the image quality of the sloped part without having to increase the memory, for example, lot of techniques have been designed to improve the jaggy of the sloped part by reducing the diameter of one pixel and moving the pixel as disclosed in Japanese Patent laid-open Application 53-52019 or Japanese Patent laid-open Application 58-150978.
FIG. 39 shows an example of a printed sloped line where the diameter of each pixel is reduced. FIG. 40 shows an example of printed sloped line where each pixel is moved. The reduction of the diameter of each pixel can be changed at five stages by changing a pulse width (laser-radiating time) of the writing laser beam. The pixel is allowed to be moved within .+-.1/3 of the diameter by changing the writing timing (laser-radiating start time) of the laser beam.
In a case that the jaggy appearing on the sloped part is improved by reducing the diameter of each pixel or moving each pixel, it is difficult to control the radiation of a beam because it needs the five-stage change of the diameter or a complicated combination of moving ranges of pixels composing the sloped part. Further, since each of the pixels composing the sloped part is substantially elliptic, for outputting an enlarged character, it is disadvantageous that the jaggy appearing on the sloped part cannot be completely removed.